Not applicable.
Not applicable.
This invention relates to a structural member and a process for producing a structural member with at least one layer of plastic and with a layer inside the structural member that has a substantially higher modulus of elasticity than plastic.
Such structural members are frequently, but not exclusively, used in the building industry. A structural member is known from German Patent DE-41 37 649 A1 which consists in general of plastic with a first low modulus of elasticity and with a layer within the building element of a material which has a second, substantially higher modulus of elasticity. This layer, which preferably consists of an aluminum alloy, has a system plane along which the profiled layer extends. For a plate-shaped element, for example, the system plane lies in a cross section of the plate-shaped structural member and the layer is constructed in a trapezoidal shape, with the layer running point-symmetrically to the system plane. The sections of the trapezoidal layer which run substantially parallel to the outer wall are connected together by sections which run diagonally, so that a section of the layer is provided reciprocally to the system plane and running substantially parallel to the outer layer. The stratification of such a layer in a structural member consisting of plastic with a low modulus of elasticity can give an increase of 5-15% in the modulus of elasticity of the structural member, for example. The sections of the layer which run diagonally and traverse the system plane then serve as shear bridges whereby the shear forces acting on the structural member can be better taken up and the modulus of elasticity of the structural member can be increased.
An I-beam is furthermore known from DE 41 37 649 A1, and is constructed analogously to the plate-shaped structural member. A layer which has a head region and a foot region connected together by a zigzag-shaped middle region is installed in the I-beam. This middle region forms a shear or thrust bridge between the head and foot regions of the layer which is installed in the structural member. Thus forces acting on the structural member can be better taken up, since an increase in the overall modulus of elasticity is attained by means of this layer, in comparison with a structural member without such a layer.
Such structural members are indeed superior to the heretofore known structural members of wood or plyboard in regard to waste disposal and recycling capability. However, the construction and development of the layers requires more processing steps for their profiling, so that production is expensive because of the bonding of the profiled layer into the structural member. The modulus of elasticity of the structural member of plastic with inlaid layers could be improved over those made of wood or by a phenolic resin bonding; however, the field of application is limited because of the relatively low modulus of elasticity of the member.
The invention therefore has as its object to provide a structural member which is easily produced and which makes possible an increase of the load-bearing ability because of a higher modulus of elasticity of the member due to the construction. A further object of the invention is to provide a process for the production of the structural member, making possible a cost-effective production of the structural member according to the invention with a high output of structural members.
These objects are achieved by a structural member with at least one system plane along which the structural member has substantially homogeneous properties and is substantially homogeneously constructed, comprising: at least one inner layer of plastic along the system plane, a respective outer layer of plastic along the system plane, and at least a respective layer of material of a substantially higher modulus of elasticity than plastic traversing the system plans and located within the structural member between the inner layer and the outer layer and arranged separated from each other. These objects are also achieved by the following process:
a) supplying an unfoamed inner layer from an extruder to a coextrusion nozzle,
b) supplying by means of the coextrusion nozzle a respective layer of substantially higher modulus of elasticity taken off from a respective sheet metal coil to an upper side and a lower side of the inner layer,
c) applying a respective outer layer by means of the coextrusion nozzle to the layers of substantially higher modulus of elasticity,
d) supplying a multilayer composite emerging from the coextrusion nozzle to a gauging,
e) withdrawing the multilayer composite from the gauging after the inner layer has foamed, and
f) cutting the multilayer composite into structural members.
The structural member designed according to the invention has the advantage that at least a 1 xc2xd times increase of the modulus of elasticity of the member can be attained, in comparison with the structural members known from the state of the art, by means of a simple arrangement and design of the layers constructed with a second, higher modulus of elasticity and built into the plastic body. It is provided therefor that two layers, which are separate from each other and which traverse the system plane, with a second higher modulus of elasticity are inserted into the structural member between an inner layer and an outer layer. These layers have respective points of intersection with the system plane which advantageously are as far apart as possible and are arranged near the wall region of the structural member. A so-called sandwich structure can thereby result, in which a core layer consisting of plastic is bounded by an upper and a lower layer with a second, higher modulus of elasticity, which in their turn can be covered with at least one outer layer consisting of plastic.
The layers with the second, higher modulus of elasticity are not connected together directly, i.e., the sections traversing the system plane and forming a shear bridge were omitted. It would have been expected, from calculations according to Steiner""s theorem, that in such a configuration a smaller load-bearing capacity or a smaller stiffness of the member would be attained, since a combined action between an upper and a lower layer would not be given by the lacking middle region or shear bridge. It was assumed that a structure with a shear bridge omitted would not act as an overall structural member, but rather as two structural members connected in parallel and only slightly affecting each other. Furthermore, it was assumed that creep would not be prevented, due to the lacking shear bridge.
Contrary to this interpretation, however, it has been found that exactly by means of this configuration, in which an upper and a lower layer are arranged separated from each other in an edge region of the outer layers of a structural member, an increase of the stiffness of the member by at least a factor of 1.5 can be made possible.
This increase of the stiffness of the member is based, according to knowledge at the time of the application, firstly on the use substantially of layers which go all through in the edge region of the structural member, and which make it possible for forces to be better taken up. In contrast to this, it is known from the plate-shaped member according to the state of the art, that because of the trapezoidal construction of the layer, only partial sections are provided which are not mutually connected in the edge region, whereby a smaller load absorption is given when bending stresses are applied. On the other hand, a combined action can nevertheless be attained, so that an increase of the modulus of elasticity of the member can be attained by the cooperation of the inner layer with the layers with higher modulus of elasticity bounding the inner layer, and the outer layers applied to these layers. It has furthermore been found that creep behavior is directly dependent on the high tensile stresses in the outermost zone and on the shear bridge formation.
An advantageous embodiment of the invention provides that the layers have a adhesion primer. A full-surface bonding between the inner layer and the outer layer to the layers with higher modulus of elasticity can thereby be made possible.
The adhesion primer, preferably applied to both sides, furthermore has the advantage that the bonding of the plastic layers and the layers which are preferably of aluminum alloy is not loosened in the cooling process during production. The thermal expansion of an aluminum alloy is smaller than that of plastic. A good bonding of the cooling plastic to the layer can nevertheless be maintained by means of the adhesion primer. A foamed inner layer is preferably used, having a degree of foaming which increases toward the middle axis, so that elastic displacements are possible which in addition contribute to the bond""s not becoming loosened.
The process according to the invention for the production of such structural members has the advantage that a continuous production of structural members is made possible in a simple manner. In this process, the materials required for the whole structural member for the formation of the individual layers of the sandwich structure can be supplied and processed simultaneously, or nearly simultaneously, through a coextrusion nozzle. The layers are then fed as a composite to a gauging step. The layers which are mutually spaced apart and are separated by an inner layer can be drawn from a coil of sheet metal and supplied to an upper side and a lower side of the inner layer. Furthermore, an outer layer can be simultaneously applied to the layers by means of the coextrusion nozzle. The coextrusion nozzle can be followed at a short distance by a gauging step. It is thereby possible for the inner layer, which can have a high foaming fraction of at least 30%, to be supplied directly to the gauging step. The foaming process thus takes place exclusively in the gauging step. The two layers, seen from a longitudinal middle axis, can be pressed into an upper and a lower edge region by the inner layer in the course of foaming, so that the intersection points of the layers with the system plane can be spaced mutually far apart.
Further advantageous embodiments and developments are given in the further claims of the description hereinafter and in the accompanying drawings.